This invention relates to systems for activating miniature markers, and more particularly to systems for excitation of resonating miniature marker assemblies for use in locating the markers in three-dimensional space.
Systems have been developed to activate and detect remote activatable marker assemblies positioned, as an example, in or on a selected item or object. The markers generate a signal used to detect the presence of the marker. Many of the activatable markers are hard-wired to a power source or other equipment external from the object. Other systems have been developed that utilize resonating leadless markers, also referred to as wireless active markers, positionable at or near a selected target. These wireless active markers are typically activated or energized by a remote excitation source that generates a strong continuous excitation signal. Accordingly, the markers generate a detectable marker signal that must be distinguished from the strong continuous excitation signal and then analyzed in an effort to try to accurately determine the target""s location. The process of distinguishing a weak marker signal from the strong continuous excitation signal, while maintaining sufficient accuracy and repeatability for determining the marker""s location, has proven to be very difficult.
Other systems have provided detection of leadless markers to determine a two-dimensional proximity (e.g., X, Y coordinates) to detectors for use with game boards, surgical tag detection devices, and medical tube placement verification systems. In the case of the game boards, a unique game piece with a resonator of a predetermined frequency is moved across the game board, and the X and Y location coordinates of the game piece relative to the game board are displayed so the players can determine the general location of the game piece on the game board. U.S. Pat. No. 5,853,327 to Gilboa identifies that the X, Y coordinates, as a function of amplitude or phase, may be determined experimentally for a given game board design. Additionally, Z distance away from the game board may be determined to a sufficient accuracy for game use by the strength of the signal above the game board provided that the signal is not a strong function of the X and Y locations. U.S. Pat. No. 5,188,368 to Ryan provides a system for determining in two dimensions which individual square of a chess board a particular chess piece is on during a chess game. The system disclosed by Ryan does not determine the Z direction.
In the case of the surgical tag and detection device, U.S. Pat. No. 6,026,818 to Blair discloses surgical devices, such as sponges, that have activatable resonator tags thereon for proximity detection by a hand-held probe. The probe has a single loop interrogation ring provided that can transmit an excitation signal to activate the resonator tag and then be switched to a receiver mode. The probe is moved manually to change the angular orientation of the interrogation ring, thereby moving the resulting excitation field""s orientation. The excitation field is moved to a suitable orientation so as to excite resonator tags in various spatial orientations. The interrogation ring can then be scanned over an area of a patient after surgery to determine if any surgical devices having the resonator tags have been left behind. Accordingly, the detection device of Blair is detecting the existence or proximity of a surgical tag with the interrogation ring, rather than the actual location of the activatable tags.
In the case of the medical tube placement verification device, U.S. Pat. No. 5,325,873 to Hirschi et al. teaches a system that detects the general position of an object within a body of tissue. The detection system includes a three-axis resonant-circuit target attached to the object and a separate hand-held detection probe having a pair of parallel and coaxially aligned transmitter/sensing coils. The transmitter sensing coils generate a current that determines whether a return signal strength of the target is great enough to be counted as a valid signal. The hand-held detection probe also has a pair of receiver coils positioned within each of the transmitter coils and connected in a series-opposed fashion. The four receiver coils allow for the creation of a null circuit condition when the target is equidistant from each of the receiver coils. The detection probe also has a visual display coupled to the receiver coils and configured to indicate the direction (e.g., left/right/up/down) in which the probe should be moved to center the detection probe over the object, thereby achieving the null circuit condition.
The systems of the above patents activate the markers with a pulsed excitation signal generated by driving an untuned source coil with either a unipolar polarity to produce a wide band impulse function or a bipolar polarity to create a waveform that more closely matches the desired resonant frequency of the marker. The required levels of magnetic excitation for the markers in the above patents are relatively low such that the excitation energy in the source coil is substantially consumed after each pulse due to the pulse circuitry resistive losses. The source coils are driven by linear amplifiers, and in one case by linear amplifiers at both ends of the coil, and by a simple pulse network that energizes the coil and extinguishes resistively. The amplitude of the pulsed excitation signal required for these applications is relatively low since either the resonator circuit to be located is of a large size, the volume in which the resonator must be located is relatively small, or the accuracy requirements locating the resonator are quite low. Accordingly, the existing systems are not suitable for use in many situations wherein highly accurate determinations of the marker""s location in three-dimensional space is required. The existing systems may also not be suitable for use with efficient, high energy systems for energizing the marker assemblies so as to provide a sufficient marker signal for use in determining the location of the marker in three-dimensional space relative to remote sensors.
Other systems have been developed for tracking wireless tags for use as a tangible interface to interact with a computer. Such systems are described in xe2x80x9cFast Multi-Axis Tracking of Magnetically Resonant Passive Tags: Methods and Applications,xe2x80x9d Kai-yuh Hsiao, Massachusetts Institute of Technology, 2001. As an example, one tracking system utilized wireless markers tracked in three-dimensional space by a single transmitter coil or by aligned multiple coils in a Hemholtz configuration. The multiple coils are continuously swept through a range of frequencies to activate the magnetically resonant passive tags, and the transmitter coils are simultaneously monitored so as to detect and determine the location of the magnetically resonant passive tags.
Other systems have been developed for proximity detection of resonator tags for Electronic Article Surveillance (EAS) systems. The requirements for EAS systems are to detect the presence of a security tag within a six-foot wide aisle using one antenna assembly for both excitation and detection of the tag within the aisle. Some EAS systems utilize tuned resonant excitation source coil drive circuitry for pulsed resonator tag operation. As an example, U.S. Pat. No. 5,239,696 to Balch et al. discloses a linear amplifier using current feedback linear power amplifiers to drive an excitation source tuned to resonant coils for use in pulsed EAS systems. The current feedback is used to adjust the linear amplifier""s drive current level provided to the tuned excitation source coil load. The current feedback is also used to provide for a relatively constant current drive for exciting resonant EAS tags in the field. The source coil is tuned to allow for use of a simple, low voltage linear amplifier circuit design. The source coil current pulse waveform is determined by the summation of the sinusoidal control signal and the drive current feedback signal input to the linear amplifier.
U.S. Pat. No. 5,640,693 to Balch et al. discloses the use of linear power amplifiers to drive excitation source coils for use in pulsed EAS systems. An apparatus for switching power to a linear amplifier is provided to turn to an xe2x80x9conxe2x80x9d state and an xe2x80x9coffxe2x80x9d state used to control the output drive pulse burst of the tuned excitation source coils. Balch et al. ""693 also identifies that linear amplifiers which generate drive signals for a source coil since linear amplifiers are typically only about thirty to forty percent efficient. The inherent inefficiency of the linear amplifier drive is improved by switching the amplifier power xe2x80x9conxe2x80x9d and xe2x80x9coffxe2x80x9d at the same time that the pulse control input signal to the power supply is switched to an xe2x80x9conxe2x80x9d and xe2x80x9coffxe2x80x9d position.
U.S. Pat. No. 5,815,076 to Herring teaches one or more damping circuits provided in series with excitation source coils and used to promote rapid dampening of the pulsed excitation interrogation signals at the end of each signal pulse. Providing the switchable damping circuits in series with the antennas increases the power dissipation of the device during pulse delivery due to added damping circuit switch resistance in the antenna current path.
The above systems employ a resonator circuit energized with an excitation signal and the resonator response signal is measured with sensing coils. The amplitude of the pulsed excitation signal required for these applications is relatively low since either the resonator circuit to be located is of a large size, the volume in which the resonator must be located is relatively small, or the accuracy requirements locating the resonator are quite low.
Under one aspect of the invention, a system is provided for generating a magnetic field for excitation of a leadless marker assembly. The system of at least one embodiment includes a source generator that generates a plurality of independently controlled, alternating electrical signals each having-an adjustable phase relative to each other. A plurality of excitation coils are independently coupled to the source generator, and each excitation coil has a coil axis axially misaligned with the coil axis of the other excitation coils. The excitation coils are configured to simultaneously receive a respective one of the alternating electrical signals at a selected phase to generate a magnetic field. The phase of the alternating electrical signal for each excitation coil is independently adjustable relative to the phase of the alternating electrical signal for the other excitation coils so as to adjust the magnetic field from the respective coil. The magnetic fields from the excitation coils combine to form a spatially adjustable excitation field for excitation of the remote leadless marker assembly.
Another embodiment includes a marker system with a switching network coupled to an energy storage device. A plurality of excitation coils are independently coupled to the switching network. Each of the excitation coils has a coil axis that is non-concentric with the coil axis of the other excitation coils. The excitation coils are configured to simultaneously receive from the switching network alternating electrical signals at a selected phase to generate a magnetic field from the respective coil. The switching network is manipulatable to independently change the phase for each excitation coil. The magnetic fields from the excitation coils are configured to combine with each other to form a modifiable excitation field having selected spatial characteristics for excitation of the leadless marker assembly. The spatially adjustable excitation field of this embodiment effectively avoids blind spots in the excitation of single axis marker assemblies and therefore allows the marker assemblies to be in any orientation relative to the pulsed source generator coil assembly and still be highly energized upon activation of the excitation source coils.
Another embodiment is directed to a marker system with a plurality of leadless resonating marker assemblies excitable by a magnetic excitation signal and being configured to generate marker resonant signals at one or more selected resonant frequencies. The excitation system is programmable and allows for sequential excitation of unique resonant frequency marker assemblies that may be positioned at different orientations relative to the source coil assembly. For example, the excitation system could excite three unique frequency resonators each of which is oriented substantially along one of the three axes (X, Y and Z) by changing the directionality state of the source coil drive before the excitation interval of the resonator of interest and providing excitation signals that are of the appropriate directionality. A plurality of location sensors are remote from the marker assemblies and configured to receive the marker signals. A source generator assembly has an energy storage device, first and second switching networks connected to the energy storage device, and first and second excitation coils. The first and second excitation coils are substantially coplanar. The first switching network and the first excitation coil are interconnected and independent of the second excitation coil. The second switching network and the second excitation coil are interconnected and independent of the first excitation coil.
Another embodiment is directed to a method of energizing a leadless marker assembly. The method includes directing alternating electrical signals via a switching network through a plurality of excitation coils that are axially misaligned relative to each other to generate a plurality of magnetic fields having selected phases so the magnetic fields combine to form a shaped excitation field. The method also includes energizing a leadless resonating marker assembly with the spatially adjustable excitation field.
Another embodiment is directed to another method of energizing a leadless marker assembly. The method includes directing alternating electrical signals having an independently adjustable phase through a plurality of excitation coils to generate a plurality of magnetic fields that combine to form a spatially adjustable excitation field. The method also includes energizing a leadless resonating marker assembly with the spatially adjustable excitation field.